This invention relates to pneumatic radial-ply runflat tires and in particular to runflat tire construction wherein a sidewall wedge insert allows for sidewall flexibility under normal-inflated operating conditions yet provides high rigidity under deflated operating conditions.
Various methods have been devised for enabling the safe continued operation of deflated or underinflated (flat) tires without damaging the tire further and without compromising vehicle handling while driving to where the tire can be changed. Loss of tire pressure can result from a variety of causes such as a deteriorated seal between the tire and rim or a tire puncture by a sharp object such as a nail.
Pneumatic tires designed for continued operation under deflated or underinflated conditions are referred to as xe2x80x9cextended mobility technologyxe2x80x9d tires or xe2x80x9cEMTxe2x80x9d tires. They are also called xe2x80x9crunflatxe2x80x9d tires, as they are capable of being driven in the flat condition. EMT (runflat) tires are designed to be driven in the deflated condition, whereas the conventional pneumatic tire collapses upon itself when subjected to a vehicle load while deflated. The sidewalls and internal surfaces of EMT tires do not collapse or buckle onto themselves. In general, the terms xe2x80x9cEMTxe2x80x9d and xe2x80x9crunflatxe2x80x9d mean that the tire structure alone has sufficient strength to support the vehicle load when the tire is operated in the deflated state. In particular, the sidewalls are reinforced to carry the tire""s load without the need for other supporting structures or devices that are disposed internal to but separate from the tire. An example of the latter internal supporting structure is shown in U.S. Pat. No. 4,059,138, entitled xe2x80x9cRun-flat Tire and Hub Therefor.xe2x80x9d
Numerous other methods and tire construction have been used to achieve workable runflat tire designs. For example, a runflat tire structural design described in U.S. Pat. No. 4,111,249, entitled xe2x80x9cBanded Tire,xe2x80x9d shows the use of a hoop or annular band approximately as wide as the tread placed under the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the deflated condition.
Generally, runflat tires incorporate reinforced sidewalls that are sufficiently rigid so as not to collapse or buckle onto themselves. Such sidewalls are thicker and stiffer than in conventional tires, so that the tire""s load can be carried by-a deflated tire without compromising vehicle handling until such reasonable time as the tire can be repaired or replaced. The methods of sidewall stiffening include the incorporation of xe2x80x9cinsertsxe2x80x9d (also called xe2x80x9cwedge insertsxe2x80x9d), which are fillers generally having a cross-sectional crescent shape. Such inserts are located in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire experiencing the greatest flex under load. The sidewalls of such tires, when operated in the deflated condition, experience a net compressive load in which the outer portions of the sidewalls are under tension due to the bending stresses while the inside portions are correspondingly in compression, especially in the region of the sidewall midway between the tire""s bead region and the ground-contacting portion of the tread.
During runflat operation (i.e. while running underinflated), due to the large mass of rubber required to stiffen and reinforce the runflat tire""s sidewalls, heat buildup from cyclical flexure of the sidewalls is a major cause of tire failure, especially when the deflated tire is operated for prolonged periods of time and at high speeds. During normal inflated operation, the hysteresis of the material of the thickened runflat tire""s sidewalls contributes to its rolling resistance, which reduces the vehicle""s fuel efficiency. The additional weight of the insert is also a disadvantage in handling and mounting a runflat tire.
U.S. Pat. No. 5,368,082 (""082) by Oare et al, having a common assignee with the present invention, disclosed the first commercially accepted runflat pneumatic radial ply tire. This patent describes the employment of sidewall wedge insert reinforcements (xe2x80x9cinsertsxe2x80x9d) to improve stiffness. The runflat tire of ""082 was generally a low aspect tire constructed with two plies, an innerliner and two inserts in each sidewall, which are disposed such that one insert is located between the two plies while the other insert is located between the innerliner and the innermost ply. Approximately six additional pounds of weight per tire was required by this low aspect ratio runflat tire to support a 362 kg. (about 800 lb.) load when deflated. This weight penalty was even more problematic when the engineers attempted to build high-aspect-ratio tires for large heavy vehicles, such as touring sedans. The supported weight for an deflated luxury car tire can exceed 453 kg. (about 1,000 lbs.). Such taller sidewalled tires, having aspect ratios in the 55% to 65% range or greater, have sidewall bending stresses that are several times that of earlier low-aspect-ratio runflat tires. The ability to handle such loads required that the sidewalls and overall tire had to be stiffened to the point of adversely affecting riding comfort and some handling characteristics. Current runflat tire design requires that there be no loss in riding comfort or vehicle handling. In the very stiff suspension performance type vehicle, such as sports cars and various sport/utility vehicles, the ability to provide such runflat tires was relatively straightforward compared to providing similar runflat tires for luxury sedans which require a softer ride. Light truck and sport utility vehicles, although not as sensitive to ride performance, provide a runflat tire market that ranges from accepting a stiffer ride to demanding the softer luxury type ride.
The runflat tire designs incorporating sidewall inserts of the sort described by Oare et al. add weight to the tire while also causing flexural heat buildup in the wedge insert material, especially during runflat operation when the magnitude of the cyclical sidewall flexure is greatest. And, as mentioned, normal-inflated riding comfort is also compromised by the additional sidewall stiffness, and the tire""s rolling resistance is greater than that of corresponding non-runflat designs. Thus, the design goals of runflat tire designers are to minimize tire weight, minimize heat buildup during runflat operation (especially at high speed) and normal-inflated operation, give minimum rolling resistance, good riding comfort and acceptable handling characteristics.
U.S. Pat. Nos. 5,427,166 (""166) and U.S. Pat. No. 5,511,599 (""599), both to Walter L. Willard, Jr., show Michelin tires that incorporate an additional third ply and a third insert in the sidewall to further increase the runflat performance of the tire over that of Oare et al. Both the ""166 and ""599 patents discuss some of the load relationships that occur in the deflated condition of the tire and demonstrate that the concept shown by Oare et al can be applied to additional plies as well as additional inserts in each sidewall. However, the use of multiple plies and inserts in each sidewall has drawbacks which include increased tire weight, increased flexure-induced heat buildup, and increased complexity in tire design, manufacturing and quality control.
A tire""s service life is dominated by normal inflated operation. Therefore, the main and most immediate design goals are good riding comfort and low rolling resistance, with tire weight being of secondary importance to the extent that it should not adversely affect the performance of sports-type vehicles. As for heat buildup, it is mostly a problem during runflat operation, being a major contributing factor to the inevitable deterioration of the tire when operated in a deflated mode.
Another example of a runflat tire design that at least partially achieves the same riding-comfort goal while also addressing the tire weight problem is covered in Patent Application Serial No. PCT/US98/13929, filed Jul. 6, 1998, and having a common assignee with the present invention. In this application, a metal-reinforced first ply carries a major part of the compressive load during runflat operation, which allows the thickness of the wedge-insert reinforcements to be less than otherwise would be needed. During normal-inflated operation, that metal-reinforced first ply experiences primarily tensile loading and also provides improved sidewall flexibility during normal inflated operation. During runflat operation, however, the metal members of the first ply undergo substantial compressive loading, especially in the sidewall portions that are most immediately adjacent to the ground-contacting portion of the tread. The tire of the invention described by the PCT/US98/13929 application addresses the design goals of full-inflated riding comfort, tire weight and extended runflat service life, but at the cost of the manufacturing penalties associated with the use of a high-modulus metal-reinforced first ply.
U.S. Pat. No. 4,779,658 discloses a tire having a pair of crescent-shaped cross-section reinforcing layers (wedge inserts), each comprising an axially outer stiffer rubber layer and an axially inner softer anticrack rubber layer. The load applied to the tire during runflat travel is supported mainly by the stiffer rubber layer, while crack generation can be reduced by the presence of the anticrack rubber layer. In the first embodiment, the anticrack rubber covers the entire inner surface of the higher elastic rubber layer. In the second embodiment, the anticrack rubber layer is only near the tire""s shoulder or above the higher elastic rubber layer to cover only the upper inclined inner surface of the higher elastic rubber layer.
European Patent No. 542,252 discloses a tire whose carcass incorporates at each sidewall a first and second reinforcing insert between the first and second ply. Against the carcass is applied an elastic support insert comprising a stiff counter core and a low modulus elastic cover.
The present invention relates to a pneumatic radial ply runflat tire having a tread, a belt structure, and a carcass comprising two beads, two sidewalls, at least one radial ply, and at least one wedge insert in each sidewall. Each insert comprises an elastomeric porous section located at the axially-inner portion of the insert and embedded in an elastomeric stiffer layer which constitutes the remainder of the insert.
The elastomeric porous section can be cross-sectionally crescent-shaped, semicircular, or triangular. The elastomeric porous section is a closed cell porous elastomer or thermoplastic elastomer. The porosity of the porous sections is between 10 and 40 percent elastomer. The elastomeric porous section can also be an open cell foam having a porosity of between 60 and 90 percent of elastomer and the remainder is gas. The porous section has a compressive modulus of between 3 MPa and 10 MPa as the section is squeezed and a compressive modulus of between 15 MPa and 80 MPa when the porous section has collapsed. The stiffer layer is a nonporous elastomer or thermoplastic elastomer having a compressive modulus of between 3 MPa and 30 MPa.
In another embodiment, each insert comprises a cross-sectionally triangular elastomeric porous section embedded in the radially-central axially-inner portion of the insert, affixed to a flexible inextensible hinge section embedded in the axially-outer portion of the insert, and two stiff wedges, stiffer than the porous section, that constitute the remainder of the insert.
When the tire is normally inflated, the porous section, and hence the insert as a whole, is flexible, thereby providing comfortable driving characteristics. Under runflat (deflated tire) conditions, the pores in the porous section collapse, and the porous layer stiffens, and the insert as a whole becomes stiff enough to support the load of the vehicle.